MNTOR-4499: subscriber id foreign key cascade for feature_flag_events

[mansaj]

References:

Jira: MNTOR-4499

Description

Add cascade update/del for feature_flag_events fk

Checklist (Definition of Done)

• Localization strings (if needed) have been added.
• Commits in this PR are minimal and have descriptive commit messages.
• I've added or updated the relevant sections in readme and/or code comments
• Product Owner accepted the User Story (demo of functionality completed) or waived the privilege.
• All acceptance criteria are met.
• Jira ticket has been updated (if needed) to match changes made during the development process.
• Jira ticket has been updated (if needed) with suggestions for QA when this PR is deployed to stage.

[github-actions]

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[Vinnl]

Since feature flags were only added recently, accounts responsible for feature flag events should always have an FxA ID.

However, since we had to import legacy unauditable feature flags, and because I wanted the author of an event to be a required column, I set the author of the legacy events to the first subscriber with an @mozilla email address. (I see now that I did not explicitly call that out in the PR, which I should've done - sorry about that.)

So what we'll probably want to do instead of adding a cascade delete, is change those to someone with an FxA ID, and then we can delete the other one without breaking any foreign keys.

[Vinnl]

Closing this, since I'm fairly sure we don't want to delete audit events. There are two other PRs option with other options, with this one I think being preferred: #5905